The present invention relates to a method for the determination of optical properties of an optical fiber, as well as the use of a method for the determination of one or more optical properties of an optical fiber. The present invention further relates to a method for selecting optical fibers for a specific configuration, particularly the selection of multimode optical fibers for use in ribbon cables, or other configurations benefitting from multimode optical fibers that are less sensitive to attenuation increase caused by bends in the fibers.
The scientific article titled “Experimental Investigation of Variation of Backscattered Power Level with Numerical Aperture in Multimode Optical Fibers”, Electronics Letters, Vol. 18, pp. 130-132, 1982, discloses experiments for the determination how the backscattered power level varies with numerical aperture. This article teaches that graded-index fibers are more sensitive to such variations than step-index fibers. The experiments disclosed therein were performed by splicing together fibers of different numerical apertures (NA) and measuring the corresponding change in backscattering level and generating two curves: a loss curve and a parameter fluctuation curve. The results obtained showed that numerical apertures variations are accompanied by Rayleigh scattering variations.
JP2008203184 relates to a method, an apparatus and a program for evaluating a characteristic of an optical fiber obtained by a bidirectional OTDR measurement from both ends of the transmission path.
JP3120437 relates to a method for measuring strain from an optical waveguide in which a reference optical fiber and a test optical fiber are connected to each other and an OTDR measuring instrument is connected to the side of the fiber. The strain of the fiber can be found by specific arithmetic from the shift and the measured Brillouin frequency shift of the fiber (t).
JP11287741 relates to a method for measuring the maximum theoretical numerical aperture of an optical fiber.
Optical fibre ribbons are used for data communication applications in which a high data speed is required. The effective high data speed is obtained by parallel transmission along a plurality of glass fibres, using a correspondingly lower speed. However, in such a situation, a delay time occurs for each optical fibre that may lead to differences in the signal arrival times between respective fibre channels. Differences in arrival times can lead to a spreading between the light pulses on the various optical glass fibres, this phenomenon being referred to as “skew”. Skew is the maximum difference in signal propagation time between the channels in an optical fibre ribbon, and is an important factor in determining the maximum speed of synchronous parallel data transmission.
Ribbon skew is expressed in the unit ps/m, being the maximum delay time difference per unit of length between the various fibers from the ribbon.
In most applications, a provision for compensation of ribbon skew is made in the receiver electronic circuit, referred to as “de-skewing”. However, the range of de-skewing can be limited, and the circuitry leads to additional costs. For this reason, it is desirable to reduce skew in the ribbon itself. Low skew ribbons with skew performances in the range of <10 ps/m to <1 ps/m are commercially available.
Factors contributing to skew in ribbons include: (1) differences in delay time per fiber; (2) differences in delay times due to differences in wavelength of the optical systems used in the various fibers of the ribbons; and (3) differences in delay time per fiber due to the ribbon making process.
Without considering the effect of the ribbon making process, the present inventors assume that the numerical aperture value tolerance is the leading contributor of delay time differences in multimode optical fibers. The maximum value for Ge-doped multimode optical fibers is in the order of 15 ps/m. A second order impact on delay time differences is in the wavelength variation of the optical signal source, which has a maximum value on the order of 2 ps/m when the total 840 to 860 nm wavelength range is applied. Differential mode delay is a third cause of delay time differences, and can be as high as several ps/m for very low bandwidth multimode optical fibers, but can be optimized down to 0.1 or 0.3 ps/m by applying high grade bandwidth multimode optical fibers. Stated in reverse, a maximum delay time difference of 1 ps/m directly leads to a minimum requirement of the effective numerical aperture (NAeff) change of less than 1% (i.e. 2×10−3) in absolute value. Assuming a normal distribution of numerical aperture values, and choosing the “+/−2σ” value as a practical measure for the extreme values, this involves a maximum statistical variance of about 2/4×10−3=0.5×10−3.
A main cause for the deviation is due to measuring inaccuracy and process tolerances. The NAeff value is measured applying the “Far Field Scanning Method”, in which a scan is made of the far field of a 2 m fiber sample illuminated by an 850 nm LED with the appropriate launching conditions. The main process influences that cause the intrinsic variation in NAeff are in: i) the Ge dope concentration variation at the core center line; and ii) the drawing induced variations in NAeff value, which are assumed to be small.
The numerical aperture values of a set of multimode optical fibers used in an optical fiber ribbon is an indicator of the skew of the ribbon. The numerical aperture values of a large batch of multimode optical fibers can be used to select multimode optical fibers for an optical fiber ribbon with low skew.
Additionally, the numerical aperture value of a multimode optical fiber is also an indicator of the macrobend sensitivity of the fiber. Macrobend sensitivity is defined as the induced attenuation, or loss in dB, when a fiber is bent to a certain bend radius over a certain number of turns. Multimode optical fibers with low macrobend sensitivity are preferred in situations where the multimode optical fiber is bent in low radii, or where the multimode optical fiber is under external stresses, such as in an optical fiber ribbon.
To determine an optical fiber delay time, also referred to as “time of flight”, which can be used to determine skew for a group of fibers and macrobend induced attenuation, specific measurement equipment is required that requires extensive operator handling. In addition, the measurement of the numerical aperture or NAeff, also requires specific measurement equipment and extensive operator handling.